Sulphur dioxide release coatings

ABSTRACT

There is disclosed a sulphur dioxide releasing film comprising: a hydrophilic polymer which is swellable upon contact with moisture; calcium sulphate or sodium sulphate; and a latent acidulant.

This invention relates to sulphur dioxide releasing films.

Perishable goods, such as foodstuffs, have a finite shelf-life due tovarious spoilage mechanisms, such as the activity of micro-organisms.There is great commercial advantage in preservation techniques forextending the shelf-life of such perishable goods.

It is known to inhibit spoilage by providing, in a suitable form, aprecursor compound which can release sulphur dioxide on contact withmoisture. European Patent Application EP 0 351 636 discloses packagingmaterial comprising two sheets of material which are laminated togetherwith a binding agent. The binding agent has dispersed therein a sulphurdioxide releasing material such as sodium metabisulphite. InternationalPatent Application WO94/10233 discloses a wide range of single andmulti-layer materials and polymeric films which are capable of releasingsulphur dioxide. However, there are a number of problems associated withthe practical implementation of these films and materials. Firstly, theprior art composites release sulphur dioxide in a manner which is notcontrolled and which is not desirable for foodstuff preservationapplications. In particular, sulphur dioxide release is spontaneous,with sulphur dioxide being released as soon as the composition isprepared. In contrast, it would be highly desirable to provide sulphurdioxide release systems which are stable on storage, only releasing thesulphur dioxide when activated by exposure to high humidity such as thatcaused by moisture release from foodstuffs. Loss of sulphur dioxideduring manufacture and, more particularly, during storage significantly(or even totally) reduces the effectiveness of the preservation system.Secondly, prior art films have a tendency to mechanically break down,particularly when exposed to high humidity levels such as thoseencountered in the vicinity of foodstuffs.

The present invention addresses the above described problems, andprovides improved sulphur dioxide releasing films which are mechanicallystable, and possess excellent sulphur dioxide release characteristics.

According to a first aspect of the invention there is provided a sulphurdioxide releasing film comprising:

a hydrophilic polymer which is swellable upon contact with moisture;

calcium sulphite or sodium sulphite; and

a latent acidulant.

Films of this kind exhibit the advantages aforesaid. The latentacidulant is a compound or compounds that are converted into acids oncontact with water, and thus supply hydrogen ions which accelerate therelease of sulphur dioxide from the sodium sulphite or calcium sulphite.The release of hydrogen ions is phased over a period of time accordinginter alia to the kinetics of the reaction of the latent acidulant withwater (which may be present as a liquid and/or as a vapour). It has beenfound that the above described combination of elements are key in orderto produce films having commercially acceptable sulphur dioxide releaseproperties. It has been found that, of the many sulphur dioxidereleasing compounds suggested in the prior art, only calcium sulphiteand sodium sulphite are acceptable, principally because it has beenfound that these compounds do not release sulphur dioxide spontaneously.In order to provide phased release of sulphur dioxide when it is wanted,ie, when moisture is present, it has been found to be essential thatcalcium sulphite or sodium sulphite is used in combination with ahydrophilic polymer and a latent acidulant. It has been found thatfavourable sulphur dioxide release profiles can be obtained, due to theinteraction of the hydrogen ion release kinetics of the latent acidulantand the moisture sensitive release characteristics of the polymer.

WO94/10233 discloses a wide range of films incorporating a sulphurdioxide releasing compound (which can be selected from a large group ofcandidates) and one or more additives from the group: acid compounds,hygroscopic compounds, polymers which degrade to produce an acid, andcompounds which become or generate an acid or acidic gas in a humidenvironment. Of the compounds which become or generate an acid or acidicgas in a humid environment, only generic examples are disclosed, and nofurther indication is provided of the identities of possible candidates.The present inventors have found that neither the specific examples northe generic combinations provided in WO94/10233 are themselves adequatefor practical usage. Furthermore, the present inventors have found thatmuch improved films can be provided using a more specific combination ofcomponents. Further still, the present inventors have found that many ofthe components taught by WO94/10233 are in fact unsuitable for use in apractical sulphur dioxide releasing film. For example, sodiummetabisulphite has been found to be highly unsuitable since sulphurdioxide is released spontaneously once the film is prepared.

The latent acidulant may comprise at least one anhydride. Lactones,lactides or other molecules that generate an acid group following areaction with water may be used instead. Typically, the reaction will behydrolysis, but alternative mechanisms are possible, for example viaoxidation reactions such as oxidation of alcohol or carbonyl compounds.The latent acidulant may comprise at least one anhydride compound fromthe group comprising succinic anhydride, benzoic anhydride, itaconicanhydride and adipic anhydride. These compounds have been found toprovide particularly good results. Surprisingly, succinic anhydride hasbeen found to provide a dual usage as a latent acidulant and as aplasticiser for the polymer. The latter property enables the provisionof films exhibiting improved mechanical properties, even in humidenvironments.

Preferably, the polymer comprises ethyl cellulose or cellulose acetate.WO94/10233 discloses an example of a film composition comprising acellulose acetate film incorporating calcium sulphite. However, thisexample lacks the element of an anhydride latent acidulant (an organicacid being employed as an acidulant) and, as a consequence, displayssulphur dioxide release characteristics which are less satisfactory thanthose exhibited by films of the present invention. Also, the mechanicalstrength of the film is not sufficient to be of practical use.Furthermore, there is no suggestion in WO94/10233 that anhydrides of thetype disclosed herein might be employed in order to produce improvedsulphur dioxide releasing films.

Alternatively, the polymer may comprise a blend of a hydrophilic polymerwith a hydrophobic polymer. An advantage associated with this approachis that a heat sealable film can be produced. The blend ratio is suchthat the polymer blend is hydrophilic and swellable upon contact withmoisture. The hydrophobic polymer may comprise polyethylene and thehydrophilic polymer may comprise poly(ethylene oxide) or poly(vinylalcohol-ethylene). Such polymer blends exhibit good mechanicalproperties and, importantly, good swelling properties.

The polymer may be capable of absorbing during swelling at least 2%,preferably at least 4%, most preferably about 10% water by weight of thepolymer.

According to a second aspect of the invention there is provided asulphur dioxide releasing article comprising a sulphur dioxide releasingfilm of the first aspect of the invention. The sulphur dioxide releasingarticle may comprise an article coated with the sulphur dioxidereleasing film. Alternative ways of combining the film with the articlemight be employed, such as by heat sealing, or even by packaging thefilm with the article.

The article may be packaging material, such as a flexible sheet materialor a container.

The article may be a foodstuff, which may be coated with the film.

Embodiments of films and articles in accordance with the invention willnow be described with reference to the accompanying drawings, in which:

FIG. 1 shows total viable counts (TVC) as a function of time on porksteak samples;

FIG. 2 shows lactic acid bacteria counts as a function of time on porksteak samples;

FIG. 3 shows pseudomonas spp. counts as a function of time on pork steaksamples;

FIG. 4 shows total viable counts (TVC) as a function of time on vacuumpacked pork slices;

FIG. 5 shows lactic acid bacteria counts as a function of time on vacuumpacked pork slices;

FIG. 6 shows pseudomonas spp. counts as a function of time on vacuumpacked pork slices;

FIG. 7 shows total viable counts (TVC) as a function of time onstrawberries;

FIG. 8 shows enterobacteriaceae counts as a function of time onstrawberries;

FIG. 9 shows total viable counts (TVC) as a function of time for 100 μmcoated films on aerobic wrapped pork slices;

FIG. 10 shows enterobacteriaceae counts as a function of time for 100 μmcoated films on aerobic wrapped pork slices;

FIG. 11 shows pseudomonas spp. counts as a function of time for 100 μmcoated films on aerobic wrapped pork slices;

FIG. 12 shows lactic acid bacteria counts as a function of time for 100μm coated films on aerobic wrapped pork slices;

FIG. 13 shows the pH of the −1 dilution for 100 μm coated films onaerobic wrapped pork slices;

FIG. 14 shows total viable counts (TVC) as a function of time for 300 μmcoated films on aerobic wrapped pork slices

FIG. 15 shows enterobacteriaceae counts as a function of time for 300 μmcoated films on aerobic wrapped pork slices;

FIG. 16 shows pseudomonas spp. counts as a function of time for 300 μmcoated films on aerobic wrapped pork slices;

FIG. 17 shows lactic acid bacteria counts as a function of time for 300μm coated films on aerobic wrapped pork slices; and

FIG. 18 shows the pH of the −1 dilution for 300 μm coated films onaerobic wrapped pork slices.

There are a number of problems which must be overcome in order toproduce a practical and commercially viable sulphur dioxide releasingfilm. Firstly, desirable release characteristics must be achieved, sothat sulphur dioxide is released in a phased manner once the film is incontact with a moist atmosphere such as that associated with foodproducts. Furthermore, it is a prerequisite that very little or nosulphur dioxide is released under relatively dry storage conditions,such as extended warehouse storage. Secondly, the mechanical propertiesof the film must be commensurate with commercial use. In particular,many films crack and peel when in contact with moist food products. In acomparative example provided below, it is shown that the example film ofWO94/10233 which displays greatest similarity to the films of thepresent invention exhibits some of the disadvantages discussed above.

A preferred polymer is ethyl cellulose. Ethyl cellulose exhibits goodswelling properties of ca. 10% swelling. Such swelling on contact withwater permits regulation of the sulphur dioxide release. Other swellablepolymers are also suitable for use: for example, cellulose acetate, orpolymer blends of polyethylene with poly(ethylene oxide) or poly(vinylalcohol-ethylene).

Succinic anhydride, benzoic anhydride, itaconic anhydride or adipicanhydride are incorporated into the film. It is possible to utilisecombinations of these anhydrides. The anhydrides act as latentacidulants. It is believed that the mechanism by which the anhydridesassist in the release of sulphur dioxide is via hydrolysis of theanhydride moiety to produce the corresponding organic acid. The organicacid liberates hydrogen ions which accelerate the hydrolysis of thesulphur dioxide releasing compound (either sodium sulphite or calciumsulphite). Thus, the kinetics of the release mechanism are very muchfavoured by the high relative humidity encountered within foodpackaging, and not favoured by dry storage conditions. An additional,and surprising, effect of the anhydride is to enhance the mechanicalstability of the film by acting as a plasticiser. The effect isparticularly marked with succinic anhydride. For example, it should benoted that ethyl cellulose coatings crack easily when spread without theanhydride additive. Such cracking was observed with pure ethyl cellulosecoatings, and with ethyl cellulose coatings having sodium sulphite as anadditive. The incorporation of various plasticising agents into suchfilms did not provide a solution to the problem. However, films whichincorporated succinic anhydride resulted in smooth and consistentcoatings.

To avoid doubt, all of the anhydride and sodium sulphite or calciumsulphite concentrations referred to below are with respect to the dryweight of polymer.

In representative but non-limiting examples, a sodium sulphite orcalcium sulphite concentration in the range 15 to 100%, preferably 25 to80%, most preferably about 30% to about 75%, is used. The concentrationof anhydride is generally in the range 10 to 100%. In the case ofsuccinic anhydride, the concentration is generally in the range 10 to50%, preferably 15 to 45%, most preferably about 30% to about 40%. Whenethyl cellulose is used, a range of ethyl cellulose viscosities aresuitable. Viscosities of 4 cps (centipose) to 300 cps have beensuccessfully employed, with higher viscosities (generally viscositiesgreater than 150 cps) being preferred since improved mechanicalproperties are associated with the film. However, lower viscositycelluloses can be useful for applications in which the film is printed.The skilled person will appreciate that in addition to printing, manyother coating methodologies might be employed, such as casting, mouldingor extrusion. Cellulose films are generally produced by casting asolution of the relevant precursor dissolved in a suitable carrier, suchas ethanol, followed by evaporation of the carrier. Preferred, butnon-limiting, concentrations of ethyl cellulose (or cellulose acetate)in the carrier are in the range 5 to 30% w/w, with higher concentrationsgenerally being preferred since enhanced sulphur dioxide releaseprofiles, in which sulphur dioxide release is sustained over an extendedperiod of time, can be obtained. The skilled reader will appreciate thatthe upper limited of the concentration for any given formulation isgenerally dictated by viscosity constraints.

Representative but non-limiting coating thicknesses are in the range 3to 15 thousandths of an inch.

EXAMPLE 1

A formulation comprising 20% succinic anhydride and 30% micronisedsodium sulphite suspended in a 27% w/w ethyl cellulose/ethanol solutionwas produced. Low viscosity (4 cps) ethyl cellulose was employed.Coatings of 100 μm and 30 μm thickness were produced, corresponding toca. 20 gm⁻² and 50gm⁻² of coating, respectively. Sulphur dioxide releasewas monitored by placing a 10 cm² piece of film and a sample ofuniversal indicator paper in a sealed container under dry and wetconditions. Table 1 shows the results of an 85 day study. Table 1indicates that under dry conditions, both coatings provide a neutral pHon the indicator paper, showing that no significant release of (acidic)sulphur dioxide occurs over the observed period of time. This is animportant consideration from a commercial perspective since storage foran extended period would be expected. Furthermore, under wet conditionssustained sulphur dioxide release is observed, as evidenced by lowobserved pH values. TABLE 1 pH values as a function of time for Example1 films Coating DAY Thickness/μm Conditions 1 2 5 6 7 8 9 12 14 17 18 21100 Wet 4 4 4 4 4 4 4 4 4 4 4 4 100 Dry 7 7 7 7 7 7 7 7 7 7 7 7 300 Wet4 4 4 4 4 4 4 4 4 4 4 4 300 Dry 7 7 7 7 7 7 7 7 7 7 7 7 Coating DAYThickness/μm Conditions 22 25 29 36 43 50 57 64 71 78 85 100 Wet 4 4 4 44 4 4 4 5 7 7 100 Dry 7 7 7 7 7 7 7 7 7 7 7 300 Wet 4 4 4 4 4 4 4 4 4 45 300 Dry 7 7 7 7 7 7 7 7 7 7 7

EXAMPLE 2

Films were prepared using ethyl cellulose of different viscosities,namely 20, 100 and 300 cps ethyl cellulose. The polymer concentrationwas 11%. In each instance, films were produced comprising succinic acidand 30% sodium sulphite, and a wrapping trial was performed over aperiod of ten days. Qualitative analysis revealed that the film producedusing 20 cps ethyl cellulose underwent mechanical breakdown within threedays. Over the ten day period, small cracks were seen in the filmproduced using 100 cps ethyl cellulose. The film produced using 300 cpsethyl cellulose exhibited strong resistance to mechanical breakdown overa period of ten days.

EXAMPLE 3

Films were produced using 300 cps ethyl cellulose in ethanol at variousconcentration levels. These films contained succinic anhydride andsodium sulphite in various concentrations. Details of the compositionsof the films are shown in Table 2. Table 3 depicts the results of aneighteen day study into sulphur dioxide release under wet conditionsusing the pH measuring method generally described in Example 1. Filmscorresponding to a relatively low ethyl cellulose concentration of 6%provide relatively rapid sulphur dioxide release, with the supply ofsulphur dioxide being virtually exhausted after five days. The rapidsulphur dioxide release appears to be independent of the amount ofactive ingredients (ie, anhydride and sulphur dioxide releasingmaterial) present. It should be noted that in further tests performed indry conditions, no sulphur dioxide was detected.

The film corresponding to an intermediate ethyl cellulose concentrationof 8.5% displayed only a marginally extended sulphur dioxide releasetime profile in comparison to the low ethyl cellulose concentrationfilms. The higher ethyl cellulose concentration films (in which 11% w/wethyl cellulose in ethanol was used) exhibited sulphur dioxide releaseover a greatly extended period of fifteen days. The concentration of 11%is approaching the upper limit of ethyl cellulose concentration at therelatively high viscosity of 300 cps. Therefore, selection of the ethylcellulose concentration used to produce the films permits control of thetime profile for sulphur dioxide release. TABLE 2 Key showing filmformulations Polymer Concentration Succinic anhydride Sodium sulphiteCoating % % % Fl 6 30 45 F2 6 40 60 F3 6 50 75 F4 6 60 90 F5 8.5 20 30F6 11 20 30

TABLE 3 pH values obtained from films of Table 2 Coating Day 1 2 3 4 5 67 8 9 F1 4 4 5 7 Ceased F2 4 4 5 7 Ceased F3 4 4 5 7 Ceased F4 4 4 5 7Ceased F5 4 4 4 4 6 7 7 F6 4 4 4 4 4 4 4 10 11 12 13 14 15 16 17 18 F1F2 F3 F4 F5 7 7 7 7 7 7 7 F6 4 4 4 4 5 6 6

EXAMPLE 4

Preservative packaging is the most effective and acceptable means ofprolonging the shelf-life of chilled meat. In aerobic environmentsPseudomonas spp. are the most common spoilage organisms. Since theyusually comprise part of the initial population on flesh food they arealso widely distributed in the environment. Food spoilage due toPseudomonads may occur in a number of ways. In meat, lipase andproteases liberate fatty and amino acids, which after metabolism by theorganism result in off-odours, off-flavours and rancidity. At the laterstages the production of extracellular slime and the development ofgrowth, which is often pigmented, becomes visible. In this example ethylcellulose films were prepared and shelf-life of pork chops wrapped inthese films was then studied.

Films were produced using 27% w/w ethyl cellulose in ethanol with 20%succinic anhydride and 30% sodium sulphite, both loadings being apercentage value by dry weight of polymer. 4 cps ethyl cellulose wasused, and films having coating thicknesses of 100 and 300 μm wereproduced.

Economy pork chops were obtained from a local supermarket and de-bonedusing sterile instruments. Individual slices approx. 1.5 cm thick andweighing ca. 90 g were wrapped in the films. These slices were thendouble wrapped in cling film. Controls were prepared using meat sliceswrapped directly in cling film.

Samples were stored at <4° and removed at 0, 7, 10, 14, 21 and 28 daysfor microbial analysis. At each time interval one pack was sacrificedfrom the control, and film tests. Three samples of tissue of ca. 10 gweight were removed with sterile instruments from each pack. Each samplewas homogenised with 9 times its weight in sterile ¼ strength Ringer'ssolution. Samples were decimally diluted as appropriate.

Total viable counts (TVC) were made on Plate Count Agar (PCA, Oxoid) at30° C. for 48 h. Lactic acid bacteria were determined on de En, Rogosa,Sharpe agar (MRS) incubated at 30° C. for 48 h. Pseudomonas spp. werecounted on Pseudomonas Agar Base (PAB, Oxoid) supplemented withcetrimiide fucidin cephaloridine (C-F-C, Oxoid) and incubated at 30° C.for 48 h.

Microbial analysis was performed in triplicate and the average valuesare shown in FIGS. 1, 2 and 3.

The data in FIGS. 1, 2 and 3 show the effects of 4 weeks of pork steakstorage in films containing the active ingredients at the two coatinglevels.

The TVC lactic acid bacteria (LAB) and Pseudomonad profiles for the porkwrapped in cling film show no discernable lag phase and reach maximumcell populations in 10 days. The lactic acid bacteria initiallyrepresent 0.04% of the total population. At 10 days this has increasedto 29% of the total cell population. However, the dominant species at 10days are Pseudomonads.

The TVC, LAB and Pseudomonad profiles for the pork wrapped in 100 μmfilms are broadly similar to the control, although some suppression ofgrowth is observed with the 100 μm film, particularly within the first15 days. The TVC and pseudomonas counts for pork wrapped in 300 μmdecreased 2.5 log cycles in the first 24 h and remained at this levelfor 14 days. An increase in TVC and Pseudomonas was observed after 14days.

The only bacteria demonstrating growth with the 300 μm films were thelactic acid bacteria. This group of bacteria decreased 1 log cyclesfollowing the initial packing but subsequently increased. From Day 10the growth rate declines in all packs (control, 100 μm and 300 μm) andthey enter a stationary phase.

The pH of the meat at Day 28 in all cases is pH 6.7±0.1. This is a highpH for meat and indicates that Gram negative bacteria dominate. Themicrobial analysis confirms this since the pseudomonas spp. are 2-3 logcycles higher than the LAB. Meat dominated by LAB is normally pH 5.5-5.6

The Institute of Food Science and Technology recommended maximum valuefor raw meat is 10⁷ cfu/g. Above this value meat is considered unfit forhuman consumption. In this example the control pack rapidly exceededthis value (1-2 days) whilst the 300 μm packs did not reach this valuefor 21 days, extending the shelf-life by 20 days. This has been achievedby inhibition of Gram negative organisms such as Pseudomonas fragi andAlteromonas putrefaciens.

EXAMPLE 5

The shelf-life of refrigerated meat may be extended to several weeks byvacuum-packing it in bags of plastic materials of low permeability togases. These materials restrict the flow of gases to such an extent thatthe surrounding atmosphere of the meat becomes depleted in oxygen (often<1% v/v) and enriched in carbon dioxide (>20%). In these conditions theaerobic organisms responsible for the spoilage in refrigerated meatstored in air are inhibited by the carbon dioxide, and lactic acidbacteria (LAB) become the dominant group after storage. Other organismssuch as Brochothrix thermosphacta and enterobacteriaceae have also beendetected but their numbers are small.

Vacuum-packaging of meat is an established method of prolongingshelf-life and with good hygiene practice pork can be kept for up to 8weeks under commercial conditions. Spoilage is normally evident in porkas ‘sour’, ‘cheesy’, or ‘acid’ off odours and flavours which have beenattributed to short-chain fatty-acid end products of the dominant lacticacid bacteria. The shelf-life of vacuum packed meat with high pH(usually from stressed animals) is reduced due to discolouration(greening) and/or the production of objectionable ‘putrid’, ‘sulphury’odours. These unpleasant spoilage characteristics are associated withthe growth of Gram-negative organisms, particularly strands ofEnterobacteriaeeae and Alteromonas putrefaciens, which may dominatealong with lactic acid bacteria, or even outgrow the lactic acidbacteria.

Films of the same kind as described in Example 4 were produced andutilised in the manner described below.

A block of de-boned pork was obtained 72 h post slaughter. Individualpork slices approximately 1 cm thick and weighing approximately 90 gwere wrapped in films of thicknesses of 100 μm and 300 μm. These sliceswere then inserted into Cryovac pouches (BBL4) and then vacuum packedand sealed. Controls were prepared using film with no active ingredientand meat slices packed directly into the plastic pouches.

Samples were stored at <4° C. and removed at 0, 7, 21, 30, 44 and 61days for microbial analysis. At each time interval one pack wassacrificed from the Control, Control film, 100 μm and 300 μm 12 tests.Three samples of tissue of ca. 10 g weight were removed wit sterileinstruments from each-pack. Each sample was homogenised with nine timesits weight in sterile Ringer's solution (¼ strength). Samples weredecimally diluted as appropriate. Total viable counts (TVC) were made onPlate Count Agar (PCA, Oxoid) at 30° C. for 48 h. Lactic acid bacteriawere determined on de Man, Rogosa, Sharpe agar (NM) incubated at 30° C.for 48 h. Pseudomonas spp. were counted in Pseudomonas Agar Base (PAB,Oxoid) supplemented with cetrimide fucidin cephaloridine (C-F-C, Oxoid)and incubated at 30° C. for 48 h.

The average values for Total viable counts (TVC), Lactic acid bacteria(LAB) and Pseudomonas spp. are shown in FIGS. 4, 5 and 6. The data shownin these Figures demonstrates the effect of storing pork slices for 61days in films containing the active ingredient at coating levels 100 μmand 300 μm under a vacuum packed environment.

FIG. 4 shows that bacterial growth was not inhibited by themicro-aerophilic environment in the Control or Control film vacuumpacks, the growth reaching a maximum cell population of 10¹⁰ cfu/g at 44days. In contrast growth was inhibited in 100 μm and 300 μm packs. The100 μm film inhibits bacterial growth for 44 days whilst the 300 μm filmcontinues to inhibit growth at 60 days where the final cfu/g is lowerthan the initial value.

The growth profiles for lactic acid bacteria in the Control and Controlfilms are similar, reaching cell populations of 1×10⁸ cfu/g in 21 days.The LAB count increases in the packs containing the 100 μm and 300 μmpork slices up to 21 days and then declines. Growth of lactic acidbacteria in the 300 μm film packs extended the lag phase up to 7 days.Subsequent growth of LAB was slow with the cell population increasing by2 log cycles at day 61.

The profile for Pseudomonas spp. is shown in FIG. 6. The trend for theControl and Control film packs indicates that vacuum packing extends thelag phase inhibiting pseudomonas growth. Pseudomonas species are aerobicand whilst the pack atmosphere has changed some oxygen remains followingpacking. Consequently Pseudomonas spp. may be able to survive in thismicro-aerophilic environment but not increase their cell number.

The Pseudomonas profiles for the 100 μm and 300 μm film packs show a 1log cycle decline by day 61. In the 100 μm film packs this decreaseoccurs up to 30 days after this period, the cell population remainsstable. This could be a concentration effect or the development of aresistant population. The 300 μm film packs show a 2.5 log cycledecrease within 7 days, although a 1.5 log cycle increase is observedafter 7 days. It is thought that the initial decrease is due to theincreased loading of active ingredient in these films.

The Institute of Food Science and Technology recommended maximum valuefor raw meat is 10⁷ cf/g. Above this value meat is considered unfit forhuman consumption. In this study the Control and Control film packsexceeded this value within 20 days and the 100 μm packs at 61 days. The300 μm packs did not reach this value, remaining 2 log cycles below thislimit at 61 days. This indicates that films of the invention areeffective in extending shelf-life in vacuum packed meat products.

EXAMPLE 6

Films were produced using the following formulation:

Ethyl cellulose (300 cps)—11% w/w in ethanol;

Succinic anhydride—30% based upon dry weight of polymer;

Sodium sulphite—45% based upon dry weight of polymer.

Films of 50 μm thickness were produced.

Food grade paper samples were coated with films produced according tothe above formulation.

Approximately 30 g of strawberries (25-40 g) were packed in plastic cups(250 ml). Paper coated with the film was then added and the cups coveredwith cling film. Control packs were prepared by omitting the coatedpaper prior to wrapping with cling film.

Samples were stored at room temperature and removed at 0, 3, 5 and 7days for microbial analysis. At each time interval two packs weresacrificed from the Control, and coated paper tests. Approx. 10 g wereremoved with sterile instruments from each pack. Each sample washomogenized with 9 times its weight in sterile MRD. Samples weredecimally diluted as appropriate.

Total viable counts (TVC) were made on Plate Count Agar (PCA, Oxoid) at30° C. for 48 h. Enterobacteriaceae were determined on Violet Red BileGlucose Agar (VRBGA, Oxoid) incubated at 37° C. for 48 h. Yeast andmoulds were counted on Rose-Bengal Agar (RB, Oxoid) supplemented withchloroamphenicol and incubated at 30° C. for 72 h.

The average values for Total Viable Counts (TVC) and Enterobacteriaceae(Enteros) are shown in FIGS. 7 and 8.

FIG. 7 shows that bacterial growth was suppressed for up to 5 days. Atday 7 the TVC has increased to a value similar to the control. In thecontrol packs bacteria increase 2 log cycles in the first 3 days andremain at 10⁵ cfu/g thereafter. FIG. 8 shows the enterobacteriaceaeprofile. The enterobacteriaceae count decreases in the 50 μm film packs.However, as with the TVC, the number increases rapidly after day 5. Thisincrease is probably due to the active ingredient falling below the MIC,allowing growth of surviving organisms.

EXAMPLE 7

Films were coated with a formulation according to the invention whichused 11% w/w ethyl cellulose in ethanol, with an ethyl celluloseviscosity of 300 cps. The coatings comprised succinic anhydride andsodium sulphite at concentrations 30:45, 40:60 and 50:75, (in which thesuccinic anhydride concentration is expressed first, the sodium sulphiteconcentrations expressed lastly, and all concentrations are percentagesby weight of the dry weight of the polymer). Henceforth, a film having asuccinic anhydride concentration of 30% and a sodium sulphiteconcentration of 45% will be referred to as a ‘30:45’ film, with thisnomenclature being extended mutatis mutandis to films of othercompositions. For each combination of succinic anhydride and sodiumsulphite concentrations, coatings were produced of thickness 100 μm and300 μm. Pork obtained from a local supermarket was cut into individualslices approx. 1 cm thick and weighing approx. 50 g. The slices werethen wrapped in the coated films. These slices were then double wrappedin cling film. Controls were prepared using meat slices wrapped inuncoated film and double wrapped in cling film.

Samples were stored at <4° C. and removed at 0, 1, 3, 7, 10, 14 and 21days for microbial analysis. At each time interval one pack wassacrificed from the control and coated films tests. Three samples oftissue of ca. 20 g weight were removed with sterile instruments fromeach pack. Each sample was homogenised with 9 times its weight insterile MED. Samples were decimally diluted as appropriate.

Total viable counts (TVC) were made on Plate Count Agar (PCA, Oxoid) at30° C. for 48 h. Enterobacteriaceae were determined on Violet Red BileGlucose Agar (VRBGA, Oxoid) incubated at 37° C. for 48 h. Lactic acidbacteria was determined on de Man, Rogosa Sharpe agar (MRS) incubated at30° C. for 48 h at Pseudomonas spp. were counted on Pseudomonas AgarBase (PAB, Oxoid) supplemented with cetrimide fucidin cephaloridine(C-F-C, Oxoid) and incubated at 30° C. for 48 h.

Microbial analysis was performed in triplicate and the average valuesare shown in FIGS. 9 to 18.

The Total viable counts (TVC), Enterobacteriacease, Pseudomonas spp. andLactic acid bacteria for various coated films of 100 μm thickness areshown in FIGS. 9 to 12. Examination of the TVC profile shows that at Day3 the control and 40:60 films are at or approaching the critical limitof 10⁷ cfu/g. At Day 3, the 30:45 and 50:75 films are approximately 1log cycle lower than the control. However, by Day 7 all 100 μm coatedfilms exceed the critical limit. Subsequent microbial growth follows thegeneral trend displayed by the control, entering a stationary phasearound Day 10 with new growth after Day 14.

The organisms inhibited by the 30:45 and 50:75 films includeenterobacteriaceae, Pseudomonas spp. and Lactic acid bacteria. All theseorganisms show a decrease in cell number between Day 0 and Day 3. In allcases the dominant species remains Pseudomonas with LAB representingapprox. 10% of the cell population.

FIG. 13 shows the pH of the −1 dilution for each of the 100 μm coatedfilm test samples. The profiles follow the same general trend of initialdecrease in pH (increase in acidity) followed by a rise in pH (increasein alkalinity). This pattern resembles the microbial growth profile. Thechange in pH from Day 3 to Day 10 increases as the succinic anhydrideand sodium sulphite concentrations decrease.

The Total viable counts (TVC), Enterobacteriaceae, Pseudomonas spp. andLactic acid bacteria for various coated films of 300 μm thickness areshown in FIGS. 14 to 17. Examination of the TVC profiles shows thatbacterial growth was inhibited by all the films at this coating weight.At Day 7 only the 30:45 film has exceeded the 1.0×10⁶ cfu/g criticallimit. A bacteriocidal effect is observed for Enterobacteriaceac andPseudomonas spp. whilst a bacteriostatic effect is seen for LAB. This isstrongly demonstrated in the 40:60 and 50:75 films where inhibitionlasts for 7 to 10 days. The dominant species is Pseudomonas with LABrepresenting a maximum of 10% of the cell population.

FIG. 18 shows the pH of the −1 dilution for each of the 300 μm coatedfilm samples. Whilst the profiles follow the same general trendexhibited by the 100 μm coated films, namely an initial decrease in pH(increase in acidity) followed by a rise in pH (increase in alkalinity),the rise in pH occurs later than that exhibited by the 100 μm coatedfilms. This rise in pH signifies the end of the inhibition/lag phase andthe emergence of Pseudomonas spp. as the dominant species. Whilst theinhibition phase is longer than that exhibited by the 100 μm coatedfilms, the pH of the meat is not significantly lower.

Shelf-life extensions of up to 2 days were achieved with 100 μm coatedfilms of composition 30/45 and 50/75. At the higher coating level of 300μm, the shelf-life could be extended for 3 days with 30:45 and 7 dayswith 40:60 and 50:75.

COMPARATIVE EXAMPLE 8

Example 1 of WO94/10233 is a cellulose acetate film having calciumsulphite and an organic acid as additives. A film was produced using thefollowing constituents: cellulose acetate (10 g), acetone (100 ml),glycerol (1.2 g), sodium sulphite (5.0 g) and tartaric acid (6.25 g).Sodium sulphite is used in place of calcium sulphite owing to commercialavailability: otherwise film corresponds to Example 1 of WO94/10233.

The formulation was prepared and cast in a similar way to the conditionsdescribed in WO94/10233. Once the single layer film had dried, a 10 cm²piece was placed into a sealed, dry jar with universal indicator paper.Within 0.5 h the pH was recorded at a value of 4. This clearly showsrelease of sulphur dioxide to be instantaneous without s need for anymechanism of initiation such as high relative humidity. Indeed, oncontinuation overnight the pH decreased to a value of 1-0 showing notonly uninitiated release but very strong production of sulphur dioxidein an uncontrolled fashion. The sulphur dioxide release characteristicsof this film render it unsuitable for any commercial purpose.

1. A sulphur dioxide releasing film comprising: a hydrophilic polymerwhich is swellable upon contact with moisture; calcium sulphite orsodium sulphite; and a latent acidulant.
 2. A film according to claim 1in which the latent acidulant comprises at least one anhydride.
 3. Afilm according to claim 2 in which the latent acidulant comprises atleast one anhydride compound from the group comprising succinicanhydride, benzoic anhydride, itaconic anhydride, and adipic anhydride.4. A film according to claim 1 in which the polymer comprises ethylcellulose or cellulose acetate.
 5. A film according to claim 1 in whichthe polymer comprises a blend of a hydrophilic polymer with ahydrophobic polymer.
 6. A film according to claim 5 in which thehydrophobic polymer comprises polyethylene.
 7. A film according to claim5 in which the hydrophilic polymer comprises poly(ethylene oxide) orpoly(vinyl alcohol-ethylene).
 8. A film according to claim 1 in whichthe polymer is capable of absorbing during swelling at least 2%,preferably at least 4%, most preferably about 10% water by weight of thepolymer.
 9. A sulphur dioxide releasing article comprising a sulphurdioxide releasing film according to claim
 1. 10. A sulphur dioxidereleasing article according to claim 9 comprising an article coated withthe sulphur dioxide releasing film.
 11. A packaging material accordingto claim
 9. 12. A foodstuff according to claim 9.